I prefer your (cleanup of Elvee's original) way drives are
routed, over my re-drawn. Much less confusing to keep
the drives together as the pair that they are.

But I still prefer my way of drawing diodes straight down
the middle, as they are all in a series. Also current mirror
style of drawing transistors paired near the top, better
shows how the emitter voltages are forced to be equal.

Obviously old -M ref designators won't match current PCB.
If we merge these best of both ways of drawing this circuit,
I think it would become much easier to introduce to those
yet unfamiliar with its rather unusual theory of operation.

R14 (new ref) probably deserves special comment added
to the drawing, PSRR function is not at all self-evident.

I prefer your (cleanup of Elvee's original) way drives are
routed, over my re-drawn. Much less confusing to keep
the drives together as the pair that they are.

But I still prefer my way of drawing diodes straight down
the middle, as they are all in a series. Also current mirror
style of drawing transistors paired near the top, better
shows how the emitter voltages are forced to be equal.

Obviously old -M ref designators won't match current PCB.
If we merge these best of both ways of drawing this circuit,
I think it would become much easier to introduce to those
yet unfamiliar with its rather unusual theory of operation.

R14 (new ref) probably deserves special comment added
to the drawing, PSRR function is not at all self-evident.

i do realy like your drawing, i must have overlooked it when scrolling this thread. not much to improve there!
if we would merge our versions the whole readability of the original schematic would be much improved.

i now see how to group the "mid-section" with the series diodes. my PCB could be improved by that

I missed that.
I am not sure it is a good idea: without a local decoupling, it won't be possible to achieve the transient cleanliness displayed in the oscillograms at the beginning of thread.
There is still plenty of room in the middle of your PCB, and it would certainly be possible to include at least a minimal on-board decoupling: nowadays, 10µ/63V caps are tiny, and they would make a big difference if there some length of wiring to the supply caps.

If you choose to include them, place the supply pads in close proximity, to avoid creating low impedance loops in the tracks.

I reiterate also about the respective placement of the output pad and the feedback take-up point: as they currently are, they will easily cause a doubling of the THD.

I can assure you it does, and it even has a more dramatic effect than what I had first guesstimated.

I ran two simulations to compare the situations: first, a "normal" one, where the track routing is state-of-the-art, and the other one based on the actual layout, including the tracks resistances and configuration estimated from the drawing, assuming standard PCB process.

You can see the effect of several misplaced milliohms: the even order distortions shoot up by a factor of more than 10!!!!

The overall THD figure increases from ~39ppm to 289ppm.

On the first version of my prototype, the initially measured THD was in the 40ppm range.
After a careful clean-up and some component rearrangements, I arrived at 15ppm, better than in the sim.

This means that the effect of the layout is not benign at all, it can very easily ruin an otherwise excellent amplifier.
That is the reason why I am so finnicky about those seemingly secondary "details".

PS
If you tin the tracks, or if you use heavier than standard PCB copper, the observed effects will be attenuated, but they will remain significant.
Even with superconducting tracks, an inductive effect would remain. I didn't model the inductance in the sim, this means that in reality, the situation will be even worse than predicted

I hope to make a discrete amplifier some day soon. This design has peaked my interest. Aprox 15 CAD of silicon per channel and a forgiveness of build quality and part selection make this very interesting.

I have a few questions, some of them smarter then others:

1- Would a pair of MJE3055T work for power transistors? I ask because Heat sinks for TO-220 are easier to build. (I suspect the answer is "yes, but only for less then 20v rails")

2- Does the PSU need anything more fancy then transformer, rectifier bridge and caps?

3- (The newbie question) how does one turn the PNG that powerflux posted into a PCB. More to the point, how does one extract the bits that are copper and print that at the right scale? I gave a stab in the GIMP selecting various bits by colour, filling with white or black, but this seems ackward. What's more how do I get the scale exactly right? Powerflux hasn't posted dimensions.

I plan to do the laser-toner-flimsy-paper-iron-on transfer method for creating the PCB.

1- Would a pair of MJE3055T work for power transistors? I ask because Heat sinks for TO-220 are easier to build. (I suspect the answer is "yes, but only for less then 20v rails")

Yes, they would work, and you don't need to limit yourself to 20V: the dices are that of 3055's, and they share the same SOAR.
With 4Ω loads, you can go up to +/-30V safely, provided you don't use your amp all day long at full throttle with heavily compressed music: that would require impossible heatsinks, but for a normal use, that's OK.
There are many cheap, higher power plastic alternatives too: like BD249 or TIP35

Quote:

2- Does the PSU need anything more fancy then transformer, rectifier bridge and caps?

No. The PSRR is not huge, but sufficient for practical purpose, and the amp works perfectly, even with lightly filtered unregulated PSU's.

Quote:

3- (The newbie question) how does one turn the PNG that powerflux posted into a PCB. More to the point, how does one extract the bits that are copper and print that at the right scale? I gave a stab in the GIMP selecting various bits by colour, filling with white or black, but this seems ackward. What's more how do I get the scale exactly right? Powerflux hasn't posted dimensions.

I plan to do the laser-toner-flimsy-paper-iron-on transfer method for creating the PCB.

You will have to wait Powerflux is satisfied enough with his work to post the source file and/or a printable file.
I just hope he wasn't put off by my comments, that was only constructive criticism, and I appreciate any contribution, including his....